mysql: fix start script for /run on separate tmpfs

Recently the default skeleton was changed to have /run as a separate
tmpfs instead of a symlink to /tmp. /run is not world-writable, but
mysqld tries to write its PID file as user mysql. Therefore, it fails
to start.

To fix, create /run/mysql and give it to user mysql.

While we're at it, change the paths to /run instead of /var/run.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>